I guess this might be a dumb question but does FreeBSD automatically take advantage of processor specific instructions like MMX, SSE, 3DNow! etc. or do you have to explicitly add these options to the kernel and make.conf to use them?

Generally KERNEL suffers when you force it to use MMX or other complex instructions, so they are disabled, because of performance for the kernel itself for sure, even if you specify them in /etc/make.conf the build system will disable them because its no good.

On the other side userland apps like Mplayer take quite big advantage of these, so you should use them for userland apps, but I did not checked if Mplayer is build with them by default for example.

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I have no idea how old some of the mplayer documentation is or if this is even relevant anymore.

This option is very old. SSE is enabled by default now. There is an option to disable it, however. For mplayer, you can disable dynamic cpu checking in the options and it will compile for your processor only.

By default, it builds a binary that detects cpu features at run-time. This is a little slower.
If you adjust the port options, it will auto-detect the cpu features at build time, and build a custom binary for your processor.

I don't know if you can specify a different processor if you are building for another system, though.

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